1. Field of the Invention
This invention relates to a magnetic recording medium and a method for producing the same through which high reproducibility can be obtained in a wide range from a short wavelength region to a long wavelength region and on which recording of high density can be applied.
2. Description of the Related Art
Recently, magnetic recording mediums have been extensively used as a recording medium for recording a large amount of information in various applications such as audio, video, computer, and the like. The recording density of the media are accordingly expected to be improved.
Under this expectation, vertical magnetic recording is drawing attention, which makes hexagonal ferromagnetic powder such as ferrite powder in ultra fine particles whose magnetization easy axis is vertical to a plane of the ferrite particle to orient such that the particle plane is horizontal to a surface of a magnetic layer, and thereby utilizes remnant magnetization vertical to a surface of the magnetic recording medium.
These magnetic recording media having the hexagonal ferromagnetic powder mounted highly reproduce signals in a short wavelength region less than 1 micron. Whereas, it has been revealed that high reproducibility cannot be obtained in a long wavelength region.
Therefore, there is same problem that sufficient recording and reproducing cannot be obtained when the media are applied to VTR tapes for recording audio signals and color signals in a long wavelength region.
In order to solve this problem, a trial was conducted to form a double-layered structure having a lower layer consisting of Co-.gamma. ferrite powder of low coercive force and an upper layer consisting of another Co-.gamma. ferrite powder of high coercive force, and applying the upper layer to video use and the lower one to audio use. However, this trial suffers from the fact that the upper layer is likely to demagnetize under pressure, so as to reduce the high-frequency characteristics.
On the other hand, a double-layered magnetic recording medium comprising a first magnetic layer as a lower layer formed of needle-like crystal ferromagnetic powder such as metal magnetic powder and oxide magnetic powder which are advantageous for signals having a long wavelength to record and reproduce the signals, and a vertical oriented second magnetic layer as an upper layer formed of hexagonal ferromagnetic powder in which the vertical segment of remanent magnetization is more effective for signals in a short wavelength region.
However, this two-layered magnetic recording medium suffers from many problems.
For example, the optimal current magnitude of a recording/reproduced signal for the first magnetic layer mainly consisting of the needle-like crystal ferromagnetic powder and that of the second magnetic layer consisting of the vertically-oriented hexagonal ferromagnetic powder are largely different, and high reproducing cannot be obtained for a wide range from a short wavelength region to a long wavelength region.
Moreover, since the double-layered structure of this magnetic recording medium has different magnetizing orientations between in the first magnetic layer mainly consisting of longitudinally oriented needle-like crystal ferromagnetic powder and the second magnetic layer consisting of vertically-oriented hexagonal ferromagnetic powder, a simultaneous magnetic orientation and coating of both layers is very difficult. Therefore, step-by-step coating is required for separating the two-magnetic recording layer which comprises a step of making needle-like crystal magnetic powder film to be longitudinally-oriented in the surface direction to form a lower layer and a step of forming a vertically-orientated film to form an upper layer.
This step-by-step coating, however, is not likely to provide a two-layered magnetic recording medium with good surface characteristics, because the surface characteristics of the lower layer easily affect those of the upper layer. Therefore, desired characteristics cannot be obtained and a deterioration of the S/N ratio will occur.
An object of the invention is to overcome the above-mentioned problems, by providing magnetic recording media which highly reproduce signals in a wide range from a short wavelength region to a long wavelength region and also which have excellent durability and reliability.
Another object of the invention is to provide a method for producing magnetic recording media formed of multi-layers with a high reproducibility and showing good surface characteristics and excellent electromagnetic characteristics such as S/N ratio.